Protesters sing 'We Shall Overcome' outside the Georgia Board of Pardons and Paroles while a clemency hearing for death row inmate Troy Davis goes on inside in Atlanta Monday. Davis was convicted of killing a Savannah police officer in 1989 and is due to be executed on Wednesday.

John Amis/Reuters

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Atlanta

After 20 years of professing his innocence and with three scrapped execution dates behind him, Troy Davis launched a last-ditch effort on Monday to save his life ahead of a Wednesday execution date in Georgia.

As thousands – including the pope – wait in vigils around the globe, a Georgia clemency board is hearing testimony in what promises to be a contentious hearing over whether there's enough new evidence to show that Mr. Davis did not shoot and kill off-duty Savannah, Ga., police officer Mark MacPhail in 1989.

When the clemency board met Monday, it was flanked by dozens of boxes of support signatures with labels that read, “I am Troy Davis.” Some 300 vigils and protests have been held in recent days around the country, including in Times Square. Some 663,000 signatures were collected on Davis' behalf.

Over the past decade, the case, legal experts say, has become an unprecedented cause célèbre for those who want to abolish the death penalty on the groundsit cannot guarantee that innocent people won't be executed.

The federal appeals system has turned the case over exhaustively without changing the verdict, most recently when a federal court judge dismissed Davis's claim of innocence in 2010.

At the same time, seven of nine witnesses have changed or recanted their original testimony, with some saying they were pressured by police to finger Davis. Meanwhile, new questions have cropped up about a key ballistics test, and one juror said in an Amnesty International documentary that, had she known then what she knows now, Davis would not be awaiting his execution.

With such a preponderance of new evidence, the Davis case is challenging evolving views on whether those who have been sentenced to death in US courts are guilty or innocent. At the forefront is the chasm that exists between the legal power of an initial conviction – when the burden of proof is on the prosecution and reasonable doubt is sufficient for acquittal – and the subsequent doubts raised from new evidence introduced during a convict's journey to the execution chamber.

“Davis has had to show by clear and convincing evidence that he's innocent,” says Russell Covey, a law professor at Georgia State University, in Atlanta, “which is very different from having to raise a reasonable doubt about his guilt.”

While courts in the appeals process focus on a convicted person having to prove their innocence, the Georgia parole board does have more latitude when considering "executive clemency." In allowing a 90-day stay of execution for Davis in 2007, the board wrote that it would “not allow an execution to proceed in this State unless and until its members are convinced that there is no doubt as to the guilt of the accused.”

Davis was convicted in 1991 for shooting Officer MacPhail after the officer responded to pleas for help from a homeless person near a Burger King where he worked off-duty as a security guard. MacPhail, who left behind an infant child, never had a chance to draw his gun as Davis, the jury found, shot him in the chest.

Some of those who have called for clemency, including Mr. Carter, say they're not convinced of Davis' innocence. But they are convinced that the level of doubt about his guilt has risen to a point where executing him will be a serious injustice, and would only further cast doubt on the US death penalty system around the world.

“When you put [the new evidence together], it seems to be pretty clear that no jury, if presented with that information today, could conceivably find guilt beyond a reasonable doubt,” says Professor Covey.

“This case is extraordinary because there have been substantial questions of his innocence for almost a decade,” death-penalty lawyer Stephen Bright, a professor at Yale Law School, told the Atlanta Journal-Constitution. “It has attracted attention from all around the world, and the extraordinary number of people supporting him – and the prominence of some of them – is unprecedented.”

The clemency board hearing Davis's case Monday in Atlanta – the Georgia Board of Pardons and Paroles – is the same body that previously upheld the 1991 conviction, though it does have three new members. The ultimate Georgia arbiter of clemency, the board has rarely overturned a death penalty conviction.

But in the Davis case, the five board members for the first time heard testimony from a witness who said another man at the scene confessed that he actually pulled the trigger. That statement was excluded from the district court appeal last year because the man was not available to the court to rebut that testimony. The board is entitled to hear testimony without meeting that legal standard.

In recent years, appellate courts have continued to set high bars for proving innocence. Proponents say it's the multilevel appeals processes like the one Davis has gone through that guarantee the fairness of the system. The fact that some death-penalty convictions do get overturned on appeal, or that new evidence, such as DNA proof culled by the Innocence Project, has set dozens of men free in recent years, is actually proof to many that the system works.

Yet the Davis case could also play into growing doubt, at least in some quarters, about the guarantees of the death penalty system. While public support for the death penalty is down to 64 percent from a high of 80 percent in 1994, according to Gallup, opinions in the last decade have stayed relatively stable, with the number of people against the death penalty dropping from 30 to 29 percent between 2009 and 2010, again according to Gallup.

On July 1, 2011, all death row inmates in Illinois were moved to regular jail cells after the state abolished the death penalty. Actual executions have been steadily dwindling even in the country's mostly Southern “eye-for-an-eye” states, where the ultimate sanction is most common. Executions in the US overall have declined from 98 in 1999 to 38 in 2010.